Method of chromizing metal



United States Patent O 3,451,843 METHOD OF CHROMIZING METAL John J. Eckenrod, Pittsburgh, Pa., assignor to Crucible Steel Company of America, Pittsburgh, Pa., 21 corporation of New Jersey No Drawing. Filed Jan. 9, 1967, Ser. No. 607,919

Int. Cl. B05c 3/09 US. Cl. 117-113 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The field of the invention is a method for chromizing ferrous or nickel-base alloys.

The prior art known to the inventor consists of the article Salt-Bath chromizing by I. E. Campbell et al. in volume 96 of the Transactions of the Electrochemical Society, pages 262 et seq. (1949). Salt-bath chromizing of various metals is described, but the author either used expensive and hard-to-handle chromous chloride or reduced chromic chloride to the chromous salt in situ, a timeconsuming (three-hour) process not suitable for commercial use.

SUMMARY OF THE INVENTION The invention consists in ititiating and sustaining a saltbath chromizing operation by incorporating into the salt bath ordinarily used, i.e., one consisting essentially of alkali-metal and alkaline-earth metal halides, about 0.5 to of ferrous halide, e.g., ferrous chloride, and adding ferro-chromium to the bath in suflicient amount for the chromizingreaction. Chromous halide, e.g., chromous chloride, is thus formed in situ and promptly used for chromizing. The invention is of use with the nickel-base alloys and the ferrous-base metals, including the plaincarbon, austenitic stainless, and ferritic stainless steels, but in view of the rapidity with which chromium diffuses into wholly ferritic materials, especially the wholly ferritic stainless steels of the kind disclosed in US. Patent No. 3,250,611, it is in the chromizing of such materials that the invention will find use to greatest advantage.

DESCRIPTION OF PREFERRED EMBODIMENT A sample of steel consisting essentially of 10.5% chromium, 0.08% maximum carbon, about 0.6% titanium,

3,451,843 Patented June 24, 1969 about 0.4% silicon, balance iron and usual impurities, in the form of a test coupon /2 in. wide, 4 in. long, and 0.05 in. thick, was chromized in accordance with the invention. That is, said coupon was immersed for five minutes in a molten salt bath maintained at 1850" F., the bath consisting of a mixture of (a) about barium chloride, about 9% sodium chloride, with (b) about 6 weight percent of ferrous chloride. Low-carbon ferrochromium was added in finely divided form (20 to 35 mesh) and in amount sufiicient to initiate and sustain the desired chromizing reaction. The immersion was done with the bath maintained under a nitrogen atmosphere, though carbon dioxide, argon, or other unreactive gas might have been used. The test coupon was then handled as is customary in salt-bath chromizing practices: withdrawn from the bath, cooled in a non-oxidizing atmosphere, washed to remove salt therefrom, and dried.

Upon being subjected to numerous tests, including the ferric chloride spot test, boiling nitric acid test, Watervapor column test, CASS test, and cyclic-dip test, the sample exhibited results equivalent to those obtained with samples chromized for the same time and temperature with the use of a conventional bath containing chromous chloride. The resulting chromium coating was in all respects similar to that obtained b the use of a conventional chromous chloride containing bath.

I claim:

1. The improvement in a process for chromizing an object of metal selected from the group consisting of ironbase and nickel-base alloys comprising the steps of immersing said object in a molten-salt chromizing bath, withdrawing said object from said bath, cooling said object in a non-oxidizing atmosphere, washing said object to remove salt therefrom, and drying said object, which consists in initiating and sustaining the chromizing reaction occurring in said bath by providing a bath consisting essentially of halides of metals of the alkali-metal and alkaline-earth groups and about 0.5 to 10% by weight of ferrous halide and adding ferrochromium thereto in amounts sufiicient to supply the chromium required for the chromizing operation.

References Cited UNITED STATES PATENTS 2,746,888 5/1956 Ross 117-1l3 X 2,872,348 2/1959 Eubank 11713O X 2,885,301 5/1959 Samuel 117-130 X 2,957,782 10/1960 Boller 117-130 X 3,203,811 8/1965 Boller 117130 X 3,205,090 9/1965 Caplow 117l13 ALFRED L. LEAVITT, Primary Examiner.

J. R. BATIEN, JR., Assistant Examiner.

US. Cl. X.R. 

